WASHINGTON — Regulations that tell consumers what it means when a product is labeled “gluten free” take effect Tuesday — a “major milestone,” says one of the leading experts on gluten disorders. “The gluten-free diet for someone with celiac disease is like insulin for diabetics,” says Dr. Alessio Fasano, director of the Center for Celiac Research at Massachusetts General Hospital and author of the book “Gluten Freedom.” The Food and Drug Administration has determined that as of Tuesday packaged food labeled gluten free or similar claims such as “free of gluten” cannot contain more than 20 parts per million of gluten. One caveat is that use of the gluten-free label is voluntary. There is no requirement that a package containing gluten must declare that. People who have the autoimmune disorder celiac disease can become sick if they eat the tiniest amount of gluten, a protein found in wheat, barley and rye. However, there are a range of other conditions set off by gluten, sensitivities that cause headaches, intestinal problems and respiratory issues. Additionally, gluten-free diets have become fashionable, which has helped prompt hundreds of new gluten-free products but also has meant that real medical problems are sometimes treated lightly, Fasano says. “For people like myself, this is a medical necessity. My diet is my medicine,” says Beth Hillson, the president of the American Celiac Disease Association. The market for gluten-free foods tops $6 billion, analysts say.